I am assuming it's a maternal thing going on for your boxer to take care of the little puppy . That is kicking in. Otherwise the female boxer would not tolerate this one bit. She is being a mom, to your baby. I have heard that they can actualy produce milk if she does this long enough.
My 11 week old shih tzu and 16 month old yorkie play together , and it sometimes gets what looks a litttle rough, they never hurt each other, but the puppy is so little and she is the instigater
I now keep them in separate rooms when we are not home> Two females try to be the ALpha. I do not know what age this starts. But, I have to supervise at all times.
I think your boxer really is being maternal towards your pupy thinking that is her baby.